Graphene Photonic Devices for Terahertz and Mid-Infrared

Author

Gao, Weilu

Date

2013-11-08

Advisor

Xu, Qianfan

Degree

Doctor of Philosophy

Abstract

Graphene and other strictly two-dimensional materials are the rising stars on the horizon of material science, condensed matter physics and engineering. The richness of optical and electronic properties of graphene attract much interest due to the exceptional high crystal and electronic quality resulting in large carrier mobility at room temperature and easily electrical control of carrier density, which find its true potential in photonics and optoelectronics. Novel graphene based broadband modulators, polarizer, active plasmonic resonators, ultra-fast lasers and etc are proposed and implemented in many literatures. Despite ample demonstrations of the true potential of graphene in optoelectronic devices, there is still unexplored region. In this thesis, we investigate the graphene photonic properties and optoelectronic devices in different regions ranging from longer wavelength terahertz frequency (THz) to shorter wavelength telecommunication frequency to reveal the whole picture of graphene.
The Drude-like intraband absorptoion (i.e. free carrier effect) in graphene plays an important role in THz region. However, the extinction ratio that can be obtained when THz waves passing through a single layer graphene is limited due to its one-atomic-layer thickness and the non-resonant nature of the intraband absorption. By incorporating resonate structures with graphene, the high extinction ratio of THz wave transmission will be achieved utilizing the high localized electric field near the graphene layer. Combining the electrically controlled carrier density in graphene, a graphene-based THz modulators with high modulation depth, fast speed can be built.
High carrier mobility of graphene at room temperature makes it a new platform for plasmonics with strong light-matter interactions, which has been theoretically proved to be able to support surface plasmon polarions (SPPs) with lower loss and higher mode confinement compared with metals. Furthermore, the electrically controlled carrier density of graphene renders it new possibility to build active plasmonic devices. Although many efforts have been done by either shaping the graphene to excite localized plamons or using near-field method to excite SPPs in continuous graphene layer in spite of low efficiency, we theoretically propose and experimentally demonstrate to utilize silicon diffractive gratings underneath the graphene to excite graphene SPPs, which can be actively controlled via back-gating structure.
The ac carrier dynamics of graphene have different contribution at different frequency range, which are investigated by incorporating graphene with resonators operating at different frequencies. In mid-infrared region the same structure as that in THz region is integrated with graphene that proves the almost complete transparency of graphene in mid-infrared region for intrinsically doping graphene while in the shorter wavelength of telecommnucations frequency, graphene is also integrated with silicon ring resonators, which shows large absorption. However, this large absorption is resulted from interband absorption that is quite different from what we have observed in THz region that is from intraband absorption. So in summary, the intraband and interband carrier dynamics of graphene will have different contributions in devices operating at different frequency region, which makes the various applications available.